Bring a pot of water to a boil for the noodles. Boil rice noodles according to package instructions (usually 2-3 minutes), drain, and set aside.
In a shallow bowl, mix cornstarch with 2 tbsp water. Dredge chicken breasts in the mixture until coated.
Heat 1/2 inch of vegetable oil in a large skillet over medium-high heat. Fry chicken until golden and cooked through (4-5 minutes per side). Transfer to a paper towel-lined plate to drain.
In a small bowl, whisk sweet chili sauce, soy sauce, rice vinegar, lime juice, and 1/2 tsp sugar until smooth.
In the same skillet, sauté bell pepper, carrot, and cucumber in remaining oil until slightly softened (2-3 minutes). Add cooked noodles and toss to combine.
Pour sauce over the noodle-vegetable mixture. Simmer 1-2 minutes until sauce thickens.
Top with crispy chicken, cilantro, chopped peanuts, and a drizzle of extra sweet chili sauce. Garnish with lime wedges.